The STEREO-SECCHI Extreme Ultraviolet Imager
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Transcript of The STEREO-SECCHI Extreme Ultraviolet Imager
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The STEREO-SECCHIExtreme Ultraviolet Imager
J-P. Wülser, J.R. Lemen,
T.D. Tarbell, C.J. Wolfson (LMSAL)
R.A. Howard, J.D. Moses (NRL)
J-P. Delaboudinière (IAS)
R. Mercier, M-F. Ravez (IOTA)
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Status Update
• Wavelength selection finalized: baseline for the “hot” channel is now Fe XV at 28.4 nm
• High performance Image Stabilization System (ISS) has been replaced by the simpler and lower cost Fine Pointing System (FPS)
• Flight mirror blanks and all parts for the EUVI Structural Model are currently being fabricated
• Structural Model tests scheduled for September– Acoustic test to verify entrance filter design– Vibration test to verify integrity of overall design
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Science Goals
The Extreme Ultraviolet Imager (EUVI) supports the STEREO-SECCHI science goals, including:
• Initiation of CMEs– Interactions of flux systems, reconnection– Role of coronal dimming
• Physical evolution of CMEs– 3-D structure, CME acceleration– Response of the low corona
• 3-D structure of Active Regions
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Main Design Features
• Normal incidence Ritchey-Crétien telescope• Multilayer coated optics, thin film filters• Heritage: EIT/TRACE• 98 mm aperture, 4 spectral channels, one in
each optical quadrant• Fine pointing system with active secondary• 2k x 2k backside illuminated CCD, 1.6” pixels• Circular full sun field of view to ± 1.7 R
• Blue LED aliveness source
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Optical System Overview
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Optical Design: Prescription
Effective focal length: 1750 mm
Distance Primary - Secondary: 460 mm
Distance Secondary - focus: 635 mm
Primary Secondary
Radius of curvature -1444 mm -892 mm
Conic constant -1.194 -8.42
Diameter 105 mm 48 mm
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Optical Design: Ray Trace Results
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Wavelength Selection, Coatings
• He II 30.4 nm: chromosphere, erupting prominences
• Fe IX 17.1 nm: high contrast in coronal loops• Fe XII 19.5 nm: “typical” quiet corona• Fe XV 28.4 nm: “hotter”, 2.5 MK corona• Baseline coating materials: MoSi/Si for 17.1,
19.5, and 30.4. Mg2Si/B4C for 28.4
• Calibration: Synchrotron at IAS
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Effective Area
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Temperature Response
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Sensitivity Comparison with TRACE
• Element comparison:– Detector: ~ 8 x higher QE than TRACE– Aperture: ~ 27 x smaller area than TRACE– Pixel area (arcsec2): ~ 10 x larger than TRACE– Pixel saturation (phot/pix): 5 x lower than TRACE– Assumes Aluminum-on-mesh entrance filters and
TRACE-like multilayer coatings
• Exposure times:– 3 x shorter than TRACE for same # photons/pixel– Min. exp. time: 40 ms (15 x shorter than TRACE)
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Entrance Filters (1)
• Protection during launch– Front door, but no vacuum chamber– Rationale: similar analysis filters survived launch
without vacuum chamber (TRACE, SXT)
• Acoustic test program before PDR• Two proven design options to mitigate risk:
– Baseline: 1500 Å Aluminum on a fine (70 lpi) mesh• TRACE heritage• Maximizes EUV throughput
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Entrance Filters (2)
– Alternate design: 1500 Å Al + 500 Å Polyimide on coarse support grid (5 mm spacing)
• EIT / EIT Calroc heritage• Potentially stronger due to Polyimide support• Reduced diffraction pattern• Lower throughput:
171 195 284 304
T = 64 % 56 % 32 % 26 %
• Analysis filters: TRACE design (size adjusted)• All filters manufactured by LUXEL
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Mechanical Design: Main Features
• Graphite/Cyanate Ester metering structure with Aluminum liner (SXI heritage)
• TRACE heritage active secondary mirror (FPS)• Mechanisms: recloseable front door (LASCO),
sector shutter, focal plane shutter (SXI), filter wheel (SXI). No focus mechanism
• Thin film filters launched at ambient pressure• Primary mirror mount: Invar bi-pods, bonded• Fully baffled
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Instrument Cross Section
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3-D View
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Primary and Secondary Mirror Mounts
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Pointing Stability
• The EUVI instrument requires a 0.8-1.2” (3) pointing stability to meet its proposed science objectives.
• The S/C is only required to meet a pointing stability of 3.8” (2)
• Pointing jitter at or near the 3.8” level would cause severe SECCHI science loss
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Effect of S/C JitterPerformance Simulated from TRACE Image
No jitterS/C jitter at spec level
(without ISS/FPS)Actual EIT image for comparison
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S/C Jitter and PSF
• Energy in central pixel drops by factor of 8
• Point sources that are two pixels apart become indistinguishable
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The EUVI Fine Pointing System
• The EUVI Fine Pointing System (FPS) bridges the gap between the EUVI pointing stability requirement and the S/C jitter specification
• Due to its limited scope, the FPS can be built with modest resources compared to the original ISS
• Main FPS features:– Improves pointing stability by a factor of 3-5– No compensation of PZT hysteresis necessary– Limited tilt range allows low voltage drivers– Simple digital control loop
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Comparison FPS - ISS
FPS ISS
Range +/- 7” > +/- 30”
Drive Voltage < 15 V > 60 V
Accuracy 0.8-1.9” p-p 0.3” p-p
Active element PZT - open loop PZT - closed loop
Electronics digital, < 1/2 board analog, 2 boards
Control software within GT read loop
Mass 0.2-0.4 kg 1.2-1.4 kg
Power < 0.5 W 1.5 W
Cost approx. 1/3 of ISS
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Optical Design Drivers
• ± 1.7 R FOV, 27.6 mm detector f = 1.75 m
• Symmetric PSF Ritchey-Crétien• Maximize focus error tolerance choose low
secondary mirror magnification (mag = 2.42)• Minimize solar energetic particle flux on CCD and
minimize stray light system fully baffled• Maximize aperture within cross sectional envelope
of heritage filter wheel mechanism
• Unvignetted FOV to 1.7 R
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Prescription DetailsSURFACE DATA SUMMARY:
Surf Comment Radius Thickness Glass Diameter Conic Cent.Obstr. OBJ Inf Inf 0 0 1 ENTRANCE FILTER Inf 152.8 107.5789 0 54 2 (Z-LOC OF SEC) Inf 122 105.1787 0 3 OUTSIDE BAFFLE2 Inf 335 103.2623 0 59.7 STO APERTURE MASK Inf 0 98 0 65 5 SPIDER MASK Inf 0 98 0 6 SPIDER MASK Inf 3 98 0 7 PRIMARY -1444 -239 MIRR 98.03406 -1.194 8 OUTSIDE BAFFLE1 Inf -99 69.4181 0 39.7 9 INSIDE BAFFLE2 Inf -122 57.52323 0 10 SECONDARY -892 221 MIRR 42.83404 -8.42 11 INSIDE BAFFLE1 Inf 359 37.48188 0 12 FILTER WHEEL Inf 54.91 28.79774 0 IMA CCD Inf 27.5046 0
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Preliminary Focus Error Budget
Error Source in Mirror Separation in Focus Location
Focus setting 0.030 mm
Structural stability• Mirror separation 0.007 mm 0.042 mm• Mirror to focus 0.007 mm
Thermal effects (+/- 20 C)• Mirror separation 0.003 mm 0.018 mm• Mirror assy. to focus 0.033 mm
Total (worst case) 0.130 mm
Note: The (geometrical) instrument PSF is smaller than one pixel at all field angles, if the focus errors is 0.130 mm or less.
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Preliminary Alignment Error Budget
Decenter Tip/Tilt
Primary mirror 0.25 mm 1 arcmin
Secondary mirror 0.25 mm 3 arcmin
CCD 0.35 mm 6 arcmin
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Optics Fabrication Flow
• Mirror blank fabricated, mirror pads bonded (LMSAL)• Mirror blank shipped to IOTA• Mirror surface ground and polished to sphere (IOTA)• Ion beam aspherization (IOTA)• Deposition of multilayer coatings (IOTA)• Mirror bonded to its mount (at IAS by LMSAL team)• Mirror set calibrated at synchrotron (IAS)• Mirror set shipped to LMSAL• Mirror set integrated into EUVI (LMSAL)